Introduction

A large number of insect pests utilise plant volatiles for host finding (Metcalf and Metcalf 1992), and there is growing interest in exploiting these cues in applied pest management. For example, adding a lure to a trap could provide an effective tool for: 1) monitoring insects at low population densities (i.e. early infestations or in eradication attempts); 2) reducing insect numbers via mass-trapping for control; 3) use in strategies such as attract-and-kill; and 4) disruption of host-finding behaviour (Rodriguez-Saona and Stelinski 2009). For thrips (Order Thysanoptera), attraction to odours has been recorded both for host-derived compounds (Koschier et al. 2000, Berry et al. 2006, van Tol et al. 2007) and from non-plant derived chemical compounds (Koschier et al. 2000, Davidson et al. 2007, Teulon et al. 2007b) .

The successful use of these kairomones in pest management is strongly dependent on the manner in which they are dispensed (Byers 1988, Heuskin et al. 2011). Semiochemicals are primarily volatile compounds which require an appropriate delivery and dispensing system to modulate their release in a manner that optimises insect attraction in the time period desired. The delivery systems used for semiochemicals in applied thrips research are often passive dispensers including open-ended glass vials (Teulon et al. 1993a, Hollister et al. 1995, Teulon et al. 2007b, Davidson et al. 2009, Wogin et al. 2010), impregnated rubber septa (Hamilton et al. 2005, Broughton and Harrison 2012), impregnated dental cotton rolls (Teulon et al.

2008b), semi-permeable polyethylene bags (M.M Davidson unpubl. data, M-C Nielsen unpubl. data, Skill et al. 2012) and semi-permeable barriers covering a reservoir containing the compound such as a sachet (Teulon et al. 2008a, Till et al. 2009, Broughton and Harrison

system), the initial amount of kairomone needed, protection needed from environmental factors and duration of experiment, rather than knowledge of ecological aspects such as optimal release rate of the kairomone lure for the target species.

Several studies have shown that release rates of semiochemicals are critical to insect response (Miller and Borden 1990, Mathieu et al. 1997, Phillips 1997, Ross and Daterman 1998, Reddy et al. 2005, Sweeney et al. 2006). To fully understand the chemical ecology of an organism under experimental conditions it is important to first quantify the release rate of the chosen kairomone compound from the passive dispenser (Byers 1988). With respect to the recognised thrips kairomone, methyl isonicotinate (MI), no published data are available on its release rate from any of the different dispensers identified above. Differences in release rates and/or, the choice of a passive dispenser may account for some of the variation previously experienced in field and greenhouse trials using MI to attract thrips. Although release rate studies, per se, do not identify the biological efficiency of a semiochemical (i.e the response), the results from release rate studies are important so that estimates can be made of the actual release rate during field and greenhouse trials and thus assist in estimating the dose the thrips are exposed to (Kraan and Ebbers 1990).

While the release rate depends on the physical characteristics of the passive dispenser (Golub et al. 1983, Hofmeyr and Burger 1995) and the physical properties of the semiochemical itself (Heuskin et al. 2011), environmental factors like air temperature and air flow also dictate the release of the semiochemical molecules into the air. Torr et al. (1997) found that the release of tsetse fly attractant from polyethylene sachets (zero-order release kinetics) increased considerably as the temperature increased from 21oC to 38oC. The important effect of the increase in release rates with increasing temperatures has been found in several other studies (Kraan and Ebbers 1990, Bradley et al. 1995, Stipanovic et al. 2004, Shem et al. 2009). Air flow also has an important effect on release rate. Kraan and Ebbers (1990) found that an increase in the air speed increased the release rate of moth sex pheromones, but concluded that this factor was not as important as temperature on the release kinetics. However, other studies have reported increased air flow can, under constant temperature, increase the release rate considerably. Bierl-Leonhardt et al. (1979) also found the release rate of a moth sex pheromone increased exponentially with an increase in temperature, but in addition was found to be proportional to the rate of air flow. Under both greenhouse and open field situations environmental factors can be very different and vary greatly both between and

within a location depending on the site, time of year and duration of the trapping period. Even in greenhouses where abiotic factors can be manipulated to a certain degree, temperature may still vary (Glenn 1984, Yunis et al. 1990, Berenguel et al. 2003) as does the internal air flow (Baptista et al. 1999, Wang et al. 1999, Tanny et al. 2005, Sase 2006). Wang et al. (1999) found that within a naturally ventilated greenhouse (tomato crop) in France, airflow within the house varied between < 0.1 m/s and 0.6 m/s depending on location within the greenhouse and outside wind speed (0.1–1.2 m/s) and direction over an 18-h period. Temperatures in greenhouses are managed to encourage optimal growth of crops, but variations of more than 10oC within a 24-h period can occur (M-C Nielsen, unpubl. data). To achieve reliable estimates of release rates under different field conditions it is therefore important to determine the relationship between release rates and environmental factors.

The aim of this chapter was to use gravimetric analysis to determine the release kinetics of MI from selected passive dispensers used for thrips kairomone work and establish its release rate under a range of environmental conditions. The passive dispensers of interest in this chapter were polyethylene bags, a commercially available sachet, a commercially available thrips lure LUREM-TR (Koppert Biological Systems, the Netherlands) and uncovered cotton dental rolls containing different amounts of MI (0.5, 1.0 or 2.5 ml), under different

temperatures (15, 25 or 35oC) and air flows (0.1–0.15 m/s or 0.25–0.3 m/s). The temperatures chosen were influenced by the range previously measured under greenhouse and field

experiments for thrips kairomone work. The airflows were dictated by the limitations of the operational parameters of the wind tunnel (minimum and maximum airflow used). The hypotheses were that MI was released at a constant rate (zero-order kinetics) and that temperature and air flow are major determinants in the release rates.